If the distribution was made directly from the 401(k), it's a distribution from an inherited 401(k), which seems to be the case here. If your share of the inherited 401(k) had been moved an inherited IRA by direct rollover, you would have received a Form 1099-R with codes 4 and G in box 7.
Because any amount of your dad's year-of-death RMD that had not been completed was not eligible for rollover to an inherited IRA, it was required to be distributed to you from the 401(k), which appears to be what was done.
